The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae include a hollow cavity and essentially stack one upon the other, forming a strong column for support of the cranium and trunk of the body. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies. The intervertebral bodies are also known as intervertebral disks and are made of a fibrous ring filled with pulpy material. The disks function as spinal shock absorbers and also cooperate with synovial joints to facilitate movement and maintain flexibility of the spine. When one or more disks degenerate through accident or disease, nerves passing near the affected area may be compressed and consequently irritated. The result may be chronic and/or debilitating back pain due to these spinal disorders.
One procedure for treating spinal disorders involves using substantially rigid plates to hold vertebrae in desired spatial relationships and orientations relative to each other. During the procedure, the spine can be approached anteriorly or posteriorly. In either case, holes are drilled and tapped in at least two of the vertebrae, to receive screws or other fasteners used to secure the plate. The holes are accurately positioned with reference to openings formed through the plate. In some cases the screws may be self-tapping. Typically the plate is curved about its longitudinal axis to facilitate contiguous surface engagement of the plate with the vertebrae. With the plate maintained against the vertebrae, the fasteners are secured within the holes. As a result, the plate maintains the attached vertebrae in a desired spacing and orientation with respect to each other.
One of the problems associated with bone fixation systems and methods is the tendency of screws or other fasteners to gradually work loose from the vertebrae after fixation of the bone plate. Slight shock or vibration of the vertebrae, due to walking, climbing stairs, or more vigorous activity by the patient following treatment increases this tendency, jeopardizing the integrity of fixation. Moreover, as the fasteners work loose, the outward protrusion of the heads over other components of the system can be a source of discomfort and present the risk of trauma to adjacent and surrounding soft tissue.